This invention relates to an improved orthopedic connector for joining a portion of a prosthesis of a selected material with a second portion of the prosthesis of a dissimilar material.
Many endoprostheses have been developed to replace many of the bones of the body which must be replaced due to disease or trauma. These prostheses must have a number of characteristics ranging from strength, durability, hardness, resilience, ability to receive tissue attachment at some locations, ability to resist tissue attachment at other locations and various other characteristics. Many of these prostheses so designed cannot possibly fill all of the characteristics necessary over the entire length of the prosthesis to achieve the desirable characteristics at various points along the length or breadth of the prosthesis.
It is desirable for a hip joint prostheses as shown in FIG. 1 to have a ball member 5 made of an alumina, cobalt chrome alloy, or some other hard material which enhances the wear characteristics of a femoral ball/cup combination attached to a prosthesis 9 to replicate the articulation of a human joint. A proximal stem 7 of the prosthesis 9 can be made of titanium, titanium alloy, nickel-chrome alloy, 316 LVM stainless steel or cobalt-chrome-molybdenum alloy. The ball member 5 can be captively or loosely held within a socket 13 which is made of an ultra high molecular weight, high-density polyethylene, alumina or other accepted biocompatible material which is fixed in the pelvis 17. A distal stem 16 can inserted into an intramedullary canal 9 of a femur bone 11.
An ideal combination of materials between the distal stem 7 and the ball member 5 has not been easily achievable because of the many disadvantages associated with the coupling of dissimilar materials or materials having different degrees of hardness. Typically, an implanting surgeon cannot necessarily select an optimum implant alloy for specific functions within the structure of a prosthesis without risking the potentially severe problems associated with self locking tapers or couplings between dissimilar metal alloys.
For example, titanium alloy is generally considered to be a very bio-compatible material. Clinical studies have shown that bone and tissue will adhere directly to the alloy without the interposition of an encapsulating fibrous layer. This phenomena has been utilized by prosthesis designers to design femoral components which are fixed to the patient without bone cement. On the other hand, titanium alloy wear characteristics are not optimal. Therefore, existing practice in this area calls for the use of femoral heads made of cobalt-chrome-molybdenum alloy, or ceramic material, which are held on the prosthesis by a self-locking taper junction or the like.
Movement between the two bodies of dissimilar material can create wear debris or metal ion release with is extremely disadvantageous to the surrounding tissue and its subsequently healing, and may affect the longevity of the implanted device.
U.S. Pat. No. 4,636,218 issued to Isamu Fukuura and Shigeo Niwa issued Jan. 13, 1987 shows such a combination of materials between a prosthesis stem and a ball member of dissimilar materials. Great lengths must be taken to use various materials of differing chemical properties which are compatible and reduce the risk of wear debris or metal ion release.
Still, the inherent dissimilarity in hardness of the materials and their differing mechanical properties create potential problems, especially in the implantation and fitting process where the implanting surgeon may choose from a variety of prostheses over the course of the operation to properly fit the prosthesis to the situation encountered. It must be remembered that often times, the surgeon must resect the top of the femur at differing locations depending upon the disease or trauma or other circumstances encountered on exposing the hip joint to be replaced.
Additionally, current surgical techniques call for the use of distal intramedullary canal spacers which are attached to the lower end of the prosthesis stem using self-locking tapers or the like incorporated to allow secure engagement between the distal end of the stem and the spacer. In such cases, the implanting surgeon may want to avoid boney ongrowth into the spacer, because such ongrowth would inadvertently affect bone remodeling and prevent removal of the prosthesis if the prosthesis should require future revision.
Again, dissimilar materials are often preferable wherein a titanium alloy prosthesis stem is much more durable. However, tissue and bone attachment may take place between the prosthesis stem and the intramedullary canal which would make future prosthesis revision more difficult. A distal spacer of a material to inhibit such ongrowth would be desirable but the problems which arise by the generation of unwanted metal wear debris and ion release when such dissimilar materials are so coupled together may prohibit such an arrangement.
There is great commercial interest in the prosthesis industry to manufacture and market improved orthopedic junctions which would eliminate the problems of using dissimilar materials, especially alloys having different degrees of hardness and yet, also allow interchangeability of various prosthesis components. There is a great interest in finding a simple and inexpensive solution to the above-mentioned problems without having to develop exotic materials or processes or exotic combinations thereof in an attempt to achieve proper coupling without jeopardizing the durability, strength, bicompatability and effectiveness of endoprostheses.
The features identified above as being desired for orthopedic junctions for implantable prostheses are all provided by the present invention.